![Trevithick high pressure engine of 1806](https://upload.wikimedia.org/wikipedia/commons/5/54/Trevithick_high_pressure_engine_of_1806.jpg)
A return connecting rod,
return piston rod or (in marine parlance) double piston rod engine
or back-acting engine is a particular layout for a
steam engine
A steam engine is a heat engine that performs mechanical work using steam as its working fluid. The steam engine uses the force produced by steam pressure to push a piston back and forth inside a cylinder. This pushing force can be trans ...
.
The key attribute of this layout is that the
piston rod emerges from the cylinder to the
crosshead
In mechanical engineering, a crosshead is a mechanical joint used as part of the slider-crank linkages of long reciprocating engines (either internal combustion or steam) and reciprocating compressors to eliminate sideways force on the piston. ...
, but the
connecting rod
A connecting rod, also called a 'con rod', is the part of a piston engine which connects the piston to the crankshaft. Together with the crank, the connecting rod converts the reciprocating motion of the piston into the rotation of the cranksh ...
then reverses direction and goes ''backwards'' to the
crankshaft
A crankshaft is a mechanical component used in a piston engine to convert the reciprocating motion into rotational motion. The crankshaft is a rotating shaft containing one or more crankpins, that are driven by the pistons via the connecting ...
. This layout is compact, but has mechanical disadvantages. Return connecting rod engines were thus rarely used.
The return connecting rod layout has two possible forms:
* The cylinder is between the crosshead and the crankshaft. This requires long connecting rods. To avoid unbalanced forces on the crosshead, these rods are usually paired and run either side of the cylinder.
* The crankshaft is between the crosshead and cylinder. This requires a paired piston rod or yoke, so as to pass around the crankshaft.
Both horizontal and vertical arrangements have used the return connecting rod layout. Vertical return connecting rod engines used the original 'vertical' layout, with the cylinder facing upwards to the crosshead.
Table and Steeple engines
![Coombes' table engine, Abergavenny steam rally 2012](https://upload.wikimedia.org/wikipedia/commons/1/14/Coombes%27_table_engine%2C_Abergavenny_steam_rally_2012.jpg)
'Table' and 'steeple' engines are vertical stationary engines with return connecting rods.
Table engines
Table engines place the cylinder above and between the crankshaft and the crosshead. They saw widespread manufacture by
Maudslay
Henry Maudslay (#Pronunciation and spelling, pronunciation and spelling) (22 August 1771 – 14 February 1831) was an English machine tool innovator, tool and die maker, and inventor. He is considered a founding father of machine tool techno ...
from around 1805 and were used to supply power to small engineering workshops. They were especially popular for driving
lineshaft
A line shaft is a power-driven rotating shaft for power transmission that was used extensively from the Industrial Revolution until the early 20th century. Prior to the widespread use of electric motors small enough to be connected directly to e ...
ing, as they could operate at a higher speed than beam engines.
Like the smaller
grasshopper beam engines, an advantage for Maudslay's table engines was that they could be made in factories as a complete unit. This included their large
cast iron
Cast iron is a class of iron–carbon alloys with a carbon content more than 2%. Its usefulness derives from its relatively low melting temperature. The alloy constituents affect its color when fractured: white cast iron has carbide impuriti ...
baseplate or table. Unlike horizontal engines and
house-built beam engines, they did not require the construction of a large and carefully aligned masonry foundation at the engine's site. Engines could also be made in advance and sold 'off the shelf'. Although this had cost savings, it was mostly beneficial because it was ''quicker'' than commissioning the building of an engine and engine house to go with it.
Steeple engines
Steeple engines place the crankshaft above the cylinder, between it and the crosshead. They use paired piston rods, two or even four, from the piston to the crosshead, to avoid the crankshaft. Another pattern used a triangular yoke on the single piston rod, allowing the crankshaft to pass through it.
Steeple engines were mostly used as marine engines.
Some American paddlewheel riverboats had especially large steeples that towered over their deckhouse.
The term 'steeple engine' was also used later to refer to
inverted-vertical tandem-compound engines, owing to their great height. These were not return connecting rod engines.
Locomotives
Trevithick's first high-pressure engines from 1801 onwards, including his locomotives, used the return connecting rod layout in both horizontal and vertical arrangements. The cylinders were embedded within the boiler, to avoid heat loss, and the short boiler also formed the frame of the engine. This made the return connecting rod a natural layout, with the crankshaft at the opposite end of the boiler from the crosshead. The paired connecting rods were relatively simple components, even though two were required. As they also allowed the crankshaft to use two simple overhung cranks on the ends of the shaft, rather than a complex
forged crankshaft with an internal crank, this was also a valuable simplification.
Other
early steam locomotives
Articles about steam locomotive
A steam locomotive is a locomotive that provides the force to move itself and other vehicles by means of the expansion of steam. It is fuelled by burning combustible material (usually coal, oil or, rarely, ...
such as
Murray
Murray may refer to:
Businesses
* Murray (bicycle company), an American manufacturer of low-cost bicycles
* Murrays, an Australian bus company
* Murray International Trust, a Scottish investment trust
* D. & W. Murray Limited, an Australian who ...
's
''Salamanca'' (1812) and
George Stephenson's
''Blücher'' (1815) and
''Locomotion'' (1825) also used return connecting rod engines. These all had heavy vertical cylinders set in tandem within the boiler barrel, driving through transverse crossheads above the locomotive, supported by swinging links. The complexity of this motion led to it being named '' 'the knitting' '' by drivers. The swinging link was simpler than
Watt
The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James Wa ...
's earlier
parallel motion
In kinematics, the parallel motion linkage is a six-bar mechanical linkage invented by the Scottish engineer James Watt in 1784 for the double-acting Watt steam engine. It allows a rod moving practically straight up and down to transmit moti ...
and did not constrain the crosshead to move in such an accurate straight line. However the additional length of the connecting rods for a return connecting rod engine, relative to the piston stroke, made this angulation effect less serious. The ''
Steam Elephant
''Steam Elephant'' was an early steam locomotive from North East England.
Historiography
An illustration of the locomotive first came to modern attention in 1931 and it was then generally assumed to be the work of George Stephenson. More recen ...
'' used a rudimentary set of slidebars to support the crosshead instead.
The use of a return connecting rod to a shaft beneath the boiler with overhung cranks also avoided the need for an internal crank. For Stephenson's designs, this crank axle would also have carried the locomotive's weight, not being merely a crankshaft, and so this avoided a particularly difficult piece of forging work.
One of the last locomotives to use return connecting rods was
Ericsson
(lit. "Telephone Stock Company of LM Ericsson"), commonly known as Ericsson, is a Swedish multinational networking and telecommunications company headquartered in Stockholm. The company sells infrastructure, software, and services in informat ...
and
Braithwaite
Braithwaite is a village in the northern Lake District, in Cumbria, England. Historically in Cumberland, it lies just to the west of Keswick and to the east of the Grisedale Pike ridge, in the Borough of Allerdale. It forms part of the civ ...
's
''Novelty'' at the
Rainhill trials.
; Abandonment of return connecting rod designs
Hedley's
''Puffing Billy'', a contemporary of ''Blücher'', avoided the return connecting rod in favour of a
grasshopper beam. The cylinders were now mounted side by side, but were still large, heavy and mounted as an integral part of the boiler. Although a
beam engine
A beam engine is a type of steam engine where a pivoted overhead beam is used to apply the force from a vertical piston to a vertical connecting rod. This configuration, with the engine directly driving a pump, was first used by Thomas Newco ...
has a similar layout to a return connecting rod engine, in that the piston rod points in one direction and the connecting rod runs backwards from this, beam engines are not considered as return connecting rod engines. Stephenson's manager
Hackworth's locomotive
''Royal George'' inverted the design to having the cylinders, still vertical, face downwards directly onto the crankshaft. By the time of
Stephenson's ''Rocket'', particularly as the new
fire-tube boiler
A fire-tube boiler is a type of boiler in which hot gases pass from a fire through one or more tubes running through a sealed container of water. The heat of the gases is transferred through the walls of the tubes by thermal conduction, heating t ...
locomotives became lighter and faster, it was recognised that vertical cylinders caused
hammer blow on the flimsy
fishbelly rails of the time. ''Rocket'' was built with cylinders inclined at 45° but was soon rebuilt to place them near-horizontally. Since then, almost all steam locomotives have had their cylinders placed close to horizontal.
Marine steam engines
Paddle ships
![Crosshead engine diagram of PS Belle](https://upload.wikimedia.org/wikipedia/commons/0/01/Crosshead_engine_diagram_of_PS_Belle.jpg)
The first
marine steam engines drove
paddlewheels. Paddles require a relatively high axle, that often also forms the crankshaft. For stability the main weight of the engine, i.e. its cylinder, is mounted low down. Later engines drove single screw propellers. These now required a low-mounted drive, but still benefited from the stability effect of a low-set engine. Such early engines, constrained by the technology of the time, worked at low boiler pressures and slow piston speeds. Together with the short piston stroke constrained by lack of space for the engine, these early engines required large diameter pistons in order to develop enough power.
Crosshead engines
Crosshead, 'double crosshead'
or 'square' engines were vertical engines analogous in layout to the table engine, with their cylinder above the crankshaft. The crosshead needed to be very wide, to allow the connecting rods to pass either side of the large cylinder, which in turn required a large supporting frame for the slidebars. They were popular for early American riverboats and their large wooden A frame crosshead supports were a distinctive feature. Larger engines became top-heavy and so the design was replaced by steeple or
walking beam
A marine steam engine is a steam engine that is used to power a ship or boat. This article deals mainly with marine steam engines of the reciprocating type, which were in use from the inception of the steamboat in the early 19th century to the ...
engines.
Steeple engines
European practice, particularly on the
Clyde, favoured
Napier Napier may refer to:
People
* Napier (surname), including a list of people with that name
* Napier baronets, five baronetcies and lists of the title holders
Given name
* Napier Shaw (1854–1945), British meteorologist
* Napier Waller (1893–19 ...
's steeple engine instead of the square engine. These were more complicated to construct and used more ironwork, but they placed the cylinder beneath the crankshaft and so were more stable in a narrow hull. Neither form was popular for sea-going vessels.
Screw propulsion
In
marine practice, the return connecting rod engine for screw propulsion was termed the ''back-acting''
(US parlance)
or ''double piston rod''
engine.
Trunk engines
John Penn patented the
trunk engine
Trunk may refer to:
Biology
* Trunk (anatomy), synonym for torso
* Trunk (botany), a tree's central superstructure
* Trunk of corpus callosum, in neuroanatomy
* Elephant trunk, the proboscis of an elephant
Computing
* Trunk (software), in re ...
in 1848. This was a design to allow a particularly short engine (measured in the direction of its piston rod), that was also able to support a large diameter piston. They were mounted transversely, usually as two cylinder engines, and used for naval ships with relatively high installed power. A trunk engine achieves its short length by having a large diameter, hollow piston rod or 'trunk'. The
gudgeon pin of the connecting rod is mounted inside this trunk, allowing the overall length of the two man components, the piston rod and connecting rod, to be telescoped together. As the trunk must be large enough to allow for the angulation of the connecting rod as the crank rotates, this design is limited to large diameter engines. It was also found that as boiler pressure increased, the large diameter seal around the trunk was increasingly prone to leaking.
Double piston rod engines
![HMS Agincourt engine](https://upload.wikimedia.org/wikipedia/commons/7/71/HMS_Agincourt_engine.jpg)
The trunk engine was largely replaced by the ''double piston-rod'' engine.
This was a return connecting rod engine, with the crankshaft between the crosshead and cylinder. Four piston rods were used to pass around the crankshaft, both above and below, and also to each side of the crank, as the crank throw was wider than the vertical spacing of the piston rods. As most of these engines were parallel compounds, two piston rods would be taken from each of the HP and LP pistons. In some engines, double rods were used from the large LP cylinder and a single rod from the HP cylinder, with a yoke to pass around the crankshaft.
Double piston rods could also be used from a simple engine, but these required an overhung gudgeon pin on the crosshead, so tended to be limited to smaller engines. An advantage of the double piston rod engine over the trunk was that both ends of the connecting rod were accessible for maintenance. One factor learned from naval use of horizontal cylinders was that, despite previous fears, there was little additional wear owing to the piston's weight resting on the cylinder.
An Admiralty committee of 1858 recommended strongly that older engine designs be abandoned in favour of rationalisation on only three designs: the single piston rod engine (the most recognisable type today), the trunk engine and the double piston rod.
A later variant of the trunk engine re-visited the return connecting rod layout as the
vibrating lever or
half-trunk engine. This was a paired engine with two short-stroke trunk engines facing outwards. Their connecting rods from the pistons led to upright 'vibrating levers' that could rock back and forth. These levers rotated a short axle shaft with further levers on it that in turn drove another pair of connecting rods and a shared central crankshaft. These complex engines were the invention of the Swedish-American engineer
John Ericsson
John Ericsson (born Johan Ericsson; July 31, 1803 – March 8, 1889) was a Swedish-American inventor. He was active in England and the United States.
Ericsson collaborated on the design of the railroad steam locomotive ''Novelty'', which com ...
and were little used outside these two countries.
Maudslay's siamese engine
Other compact alternatives to the return connecting rod or trunk engines were
Maudslay's siamese engine and the rare
annular piston engine. Like the trunk engine, these placed the gudgeon pin within the length of the piston stroke by having a pair of pistons and a T-shaped crosshead that could relocate the gudgeon pin behind its usual position.
Builders
*
Humphrys, Tennant and Dykes
Humphrys, Tennant and Dykes (later named Humphrys, Tennant and Co.) was a British engineering company based in Deptford, London, England.
History
The company was founded in 1852 by Edward Humphrys, formerly chief engineer of Woolwich Dockyard, ...
(later named Humphrys, Tenant & Co)
*
Maudslay
Henry Maudslay (#Pronunciation and spelling, pronunciation and spelling) (22 August 1771 – 14 February 1831) was an English machine tool innovator, tool and die maker, and inventor. He is considered a founding father of machine tool techno ...
*
Penn
Directly coupled pumps and blowing engines
![Allis blowing engine (New Catechism of the Steam Engine, 1904)](https://upload.wikimedia.org/wikipedia/commons/4/4c/Allis_blowing_engine_%28New_Catechism_of_the_Steam_Engine%2C_1904%29.jpg)
A
blowing engine
A blowing engine is a large stationary steam engine or internal combustion engine directly coupled to air pumping cylinders. They deliver a very large quantity of air at a pressure lower than an air compressor, but greater than a centrifugal fan ...
is a large
stationary steam engine
Stationary steam engines are fixed steam engines used for pumping or driving mills and factories, and for power generation. They are distinct from locomotive engines used on railways, traction engines for heavy steam haulage on roads, steam cars ...
directly coupled to
air pumping cylinders. They are used to provide the air blast for
blast furnace
A blast furnace is a type of metallurgical furnace used for smelting to produce industrial metals, generally pig iron, but also others such as lead or copper. ''Blast'' refers to the combustion air being "forced" or supplied above atmospheric ...
s and other forms of
smelter
Smelting is a process of applying heat to ore, to extract a base metal. It is a form of extractive metallurgy. It is used to extract many metals from their ores, including Silver mining#Ore processing, silver, iron-making, iron, copper extracti ...
. As the working cylinder and the driven load are both reciprocating pistons, they may be directly coupled by their piston rod. The connecting rod is only used to drive a flywheel whose inertia balances load through the cycle of the engine, not as an output shaft. These engines were some of the last new return connecting rod designs to be built.
The large vertical
blowing engine
A blowing engine is a large stationary steam engine or internal combustion engine directly coupled to air pumping cylinders. They deliver a very large quantity of air at a pressure lower than an air compressor, but greater than a centrifugal fan ...
illustrated was built in the 1890s by E. P. Allis Co. of
Milwaukee
Milwaukee ( ), officially the City of Milwaukee, is both the most populous and most densely populated city in the U.S. state of Wisconsin and the county seat of Milwaukee County. With a population of 577,222 at the 2020 census, Milwaukee is ...
(later to form part of
Allis-Chalmers
Allis-Chalmers was a U.S. manufacturer of machinery for various industries. Its business lines included agricultural equipment, construction equipment, power generation and power transmission equipment, and machinery for use in industrial setti ...
).
The air pumping cylinder is above the steam power cylinder and crosshead. The main force of the piston is transmitted to the air cylinder by a purely reciprocating action and the flywheels are there merely to smooth the action of the engine. The flywheel shaft is mounted below the steam piston, the paired
connecting rod
A connecting rod, also called a 'con rod', is the part of a piston engine which connects the piston to the crankshaft. Together with the crank, the connecting rod converts the reciprocating motion of the piston into the rotation of the cranksh ...
s driving downwards and backwards.
Similar pumping engines were also used in
waterworks
Water supply is the provision of water by public utilities, commercial organisations, community endeavors or by individuals, usually via a system of pumps and pipes. Public water supply systems are crucial to properly functioning societies. Thes ...
.
Inverted vertical engines had their cylinder at the top and water ram pumps at their base, or in a borehole below them. A crankshaft and flywheels were provided in the space between these, for smoother running rather than rotary power output. These were driven from the lower (pump) yoke, by short return connecting rods. Two Worth McKenzie engines of this type; a triple-expansion engine on 1895 and a duplex simple of 1906 were installed at the
Waterworks Museum, Hereford,
Hereford
Hereford () is a cathedral city, civil parish and the county town of Herefordshire, England. It lies on the River Wye, approximately east of the border with Wales, south-west of Worcester and north-west of Gloucester. With a population ...
and are preserved in steam there.
Notes
References
{{steam engine configurations
Steam engines by layout
Steam engines